rename `withOrigin` to `shiftOrigin` for WCS classes?

[beckermr]

I am working with a TanWCS and am seeing the following.

This bit of code illustrates what is going on

In [3]:     origin1 = galsim.PositionD(x=10, y=20) 
   ...:     origin2 = galsim.PositionD(x=20, y=40) 
   ...:     shift = origin2 - origin1 
   ...:     scale = 0.25 
   ...:     world_origin = galsim.CelestialCoord( 
   ...:         ra=10 * galsim.degrees, dec=12 * galsim.degrees) 
   ...:     wcs = galsim.TanWCS( 
   ...:         affine=galsim.AffineTransform( 
   ...:             scale, 0, 0, scale, 
   ...:             origin=origin1, 
   ...:             world_origin=galsim.PositionD(0, 0), 
   ...:         ), 
   ...:         world_origin=world_origin, 
   ...:         units=galsim.arcsec, 
   ...:     ) 
   ...:     print("origin1|wcs.toImage(world_origin):", origin1, wcs.toImage(world_origin)) 
   ...:     print("origin2|wcs.withOrigin(origin2).toImage(world_origin):", origin2, wcs.withOrigin(origin2).toImage(world_origin)) 
   ...:     print("shift|wcs.withOrigin(origin2).toImage(world_origin):", shift, wcs.withOrigin(shift).toImage(world_origin)) 
   ...:                                                                                                                                                                                                 
origin1|wcs.toImage(world_origin): galsim.PositionD(10.0,20.0) galsim.PositionD(10.0,20.0000000000229)
origin2|wcs.withOrigin(origin2).toImage(world_origin): galsim.PositionD(20.0,40.0) galsim.PositionD(30.0,60.0000000000229)
shift|wcs.withOrigin(origin2).toImage(world_origin): galsim.PositionD(10.0,20.0) galsim.PositionD(20.0,40.0000000000229)

Not that if I call withOrigin with where I want world_origin to point, It adds to the old origin. If instead I feed it a shift, it does what (at least) I expect.

cc @rmjarvis

[beckermr]

Further @rmjarvis pointed out that in the code for images, this is used by feeding it a delta/shift

https://github.com/GalSim-developers/GalSim/blob/releases/2.2/galsim/image.py#L1103

[rmjarvis]

Probably the upshot is that we should rename the WCS withOrigin function to be shiftOrigin, which would match what it actually does.
I think the documentation here is probably accurate. And the name is ok for LocalWCS's, since it gives them an origin when they didn't really have one. But for things that already have a non-zero origin, the name is confusing and shiftOrigin is probably more appropriate.

[rmjarvis]

Done. #1085